1. Field of the Invention
This invention is in the field of wide flat belts or webs that run over pulleys. The pulley is the element in the system that both drives the belt and causes it to run in the desired direction, and such a pulley is the subject of the invention.
Wide, flat belts find many industrial applications throughout industry, usually for the transfer of materials. In most common usage the belt is endless and is generally trained over two or more pulleys which lead it in the desired loop path. For satisfactory operation it is required that the belt remain running on the pulleys without walking off in either the left or right direction to the point where it will become damaged by contact with stationary frame members. Unfortunately, due to practical considerations in the manufacture of both belts and pulleys, dimensional inaccuracies are almost always present. Such imperfections that may go unnoticed in an open ended system become important when the error is repeated endlessly in the same direction, as in a cycling system of which the pulley/belt is an example.
Thus in any real system the pulleys will require periodic adjustment in order to prevent offrunning of the belt, with subsequent belt damage. Contact with frame members will not only damage the belt but may overload the drive system elements to the point of failure.
In spite of the difficulty in using wide and short belts they are indispensable in industry finding wide application in such diverse uses as belt sanders, material conveyors, and treadmills. Because of these commercial uses many devices have been introduced to assist in assuring that these wide belts run satisfactorily.
2. Prior Art
One way to prevent a wide belt from walking completely off one side of a pulley is to add an edge flange to the pulley. As long as the flange is high enough that the running belt cannot climb over it, then this means is effective in keeping the belt on the pulley. However the belt will be gradually damaged on the rubbing edge due to high force contact with the flange.
`V` belts, which run in deep grooves, track without problems, therefore a common practice is to attach a `V` belt to the pulley side of a wide flat belt running in a mating `V` groove in the pulley. This means takes advantage of the fact that the `V` belt is locked firmly in its groove. A common result however, is that the sideways movement of the wide flat belt is so powerful that it drags hard against the `V` belt either breaking the bond, or fracturing the flat belt in the process. When the belt is long enough rollers are sometimes used on the lower slack side to guide the belt into the desired path by bearing against the belt edges. Again, damage to the belt can and does result.
Belt training pulleys are often made with raised helical lands on the surface which appear to act on the belt by collapsing toward the pulley centerline under the belt load, thereby carrying the belt in the flexing direction toward the running centerline. To permit the device to work in both directions one half of the pulley has lands helical in one direction and the other half in the opposite. Thus the centering force is expected to be higher on the side of runoff, thereby urging the belt back toward the centerline. However the force is weak when compared to runoff forces and rarely assures proper tracking.
In cases where the belt is long when compared to its width satisfactory belt tracking is often accomplished by crowning one or both of the end pulleys upon which the belt runs. Crowning presents an angled support surface to the belt as it wanders off either to the right or left. The length and narrowness of the belt allow it to turn and meet the angled pulley surface. When this occurs the belt experiences a drag force which moves the belt back toward the running centerline of the pulley. In the case where the belt is wide when compared to its length, such turning across the width does not occur and the belt will continue to run toward one side or the other, as dictated by the belt dimensions and pulley orientation.